For media companies, the Cannes advertising festival is all about getting face time with top media buyers and advertisers. Yahoo CEO Marissa Mayer missed a golden opportunity to do just that last week.

Last Tuesday evening, Interpublic Group arranged a private dinner at the swanky L’Oasis for Mayer to meet executives from marketers such as Mondelez International, brewer MillerCoors and Greek yogurt maker Chobani. It was supposed to be a chance for Interpublic and some of its clients to get a first hand update from Mayer on what Yahoo has to offer.

But Mayer was nearly two hours late, and several dinner attendees including IPG Chief Executive Michael Roth ended up leaving before she arrived, people familiar with the matter said.

Mayer told some attendees that she had fallen asleep, some of the people said.

“If it were an investor meeting, a board meeting or a potential acquisition, I am sure she or someone from her team would have woken her up,” said one top executive in attendance. “It is another instance where she demonstrated that she doesn’t understand the value of clients, ad revenue or agencies,” the person added.

A Yahoo spokeswoman declined to comment other than to note “we value our partnership with IPG and all of our advertisers.”

Mayer’s hopes of improving Yahoo’s performance depends a great deal on improving Yahoo’s relationship with marketers and Madison Avenue. Yahoo has seen its share of the U.S. display advertising market decline over the past few years as marketers have shifted ad dollars to others such as Facebook and Google.

Wooing advertisers became a bigger part of Ms. Mayer’s job after she fired her chief operating officer Henrique de Castro, several months ago. At the time, the company said it would not fill his post and Ms. Mayer said she would focus more of her time on advertisers.